There is conventionally known a control apparatus for an internal combustion engine which estimates an air-fuel ratio in a combustion chamber based upon a ratio of an in-cylinder pressure detected at the timing as 60 degrees before TDC to an in-cylinder pressure at the timing as 60 degrees after TDC (for example, refer to Japanese Patent Laid Open No. JP-5-59986A). The control apparatus for the internal combustion engine is provided with a table for defining correlation between a ratio of the in-cylinder pressures and an air-fuel ratio in the combustion chamber for each engine operating condition to read out the air-fuel ratio corresponding to the ratio of the in-cylinder pressures from the table.
However, it is not easy to define, in detail and accuracy, correlation between a ratio of the in-cylinder pressures between two prescribed points and an air-fuel ratio in the combustion chamber for each engine operating condition. From this respect, it is difficult to actually apply the conventional control apparatus to the internal combustion engine.
Therefore, it is an object of the present invention to provide a control apparatus and method for air-fuel ratio calculation for an internal combustion engine in practical use which is capable of highly accurately detecting an air-fuel ratio in a combustion chamber.